If you're still interested in what an Intel hex file is, a little googling will locate it.

For this and other file formats, you can check out: http://www.wotsit.org/

As to how the programmer actually writes to an 8051, as Michael said each derivative is different. This is why you have to select exactly which vendor/part you wish to program, even if the code going into it is the same.

For this and any other chip-specific questions, the answer is in the datasheet.

Dennis